The present invention relates to cutting tools and, in particular, it concerns indexable cutting inserts with round or polygonal cutting edges and tool holders therefor.
It is known to employ cutting inserts which have round or polygonal cutting edges mounted in a receiving pocket of a tool holder of a cutting tool. During a cutting operation (turning, milling, etc.), only a portion of the available cutting edge typically actually cuts the workpiece. The extent of this portion depends on the depth of cut. When a portion of an indexable cutting edge of a regular (e.g., polygonal) insert has been worked, the insert is indexed to present a whole new cutting edge.
In the case of cutting inserts with polygonal cutting edges, proper operation can only be achieved by securing the insert against rotation during use. Conventional teaching has relied on abutment surfaces resulting from the inherent polygonal shape of the cutting insert. However, especially in high torque applications, or in polygons with large numbers of sides, the inherent abutment geometry may prove inadequate to withstand torques which may result from cutting forces.
Cutting inserts with round cutting edges exhibit rotational symmetry and could theoretically be continuously indexable in any angular position to accommodate actually observed wear. In practice, however, round inserts have also become restricted to use with a discrete number of indexing stations, angularly spaced by an equal shift angle. The angular shift between the indexing stations is referred to as the "angular pitch" of the insert.
In order to make optimal use of a round cutting insert, it is desirable to prevent rotation of the insert from its indexed position during the cutting process, thereby limiting the wear to a defined portion of the cutting edge. This ensures that the portion of the cutting edge presented after repositioning is, in fact, un-used.
A number of designs have been suggested for anchoring round inserts against rotation. These designs may be subdivided into two types, namely, "seat-pinning" and "lateral abutment".
Examples of a seat-pinning design may be found in U.S. Pat. No. 5,236,288 to Flueckiger and European Patent Publication No. 300,172 to Stashko.
U.S. Pat. No. 5,236,288 to Flueckiger discloses a round indexable cutting insert which has radial grooves in its base. A conically tipped screw is mounted adjustably in the seat of an insert-receiving pocket with its tip projecting so as to engage one of the radial grooves in the base of the insert.
European Patent Publication No. 300,172 to Stashko discloses a similar design in which a pin projecting from the seat of an insert-receiving pocket engages a shaped recess in the base of a round cutting insert. The shaped recess is provided either in the form of a polygonal recess with the pin engaging its corners, or as a circular recess with a number of peripheral arcuate grooves shaped to receive the pin.
Both of the above designs suffer from limited torque-resisting capability. The torque-resisting surfaces are all small, relatively close to the central axis of the insert, and are limited to a small depth close to the base of the insert. Furthermore, the orientation of the contact surfaces between the pin and the recess is not optimal for reacting against an applied torque. This combination of poor size, location and orientation of the contact surfaces results in relatively large bearing forces acting on small abutment surfaces with an accompanying risk of breakage or incomplete locking.
An example of a lateral abutment design may be found in U.S. Pat. No. 3,546,336 to Rescigno. U.S. Pat. No. 3,346,336 to Rescigno discloses a round cutting insert which is provided with five angularly spaced flat facets in its circumferential edge surface. The facets are inclined outwards towards the top of the insert such that the upper ends of the facets are spaced below the cutting edge. A fixing or clamping screw secures the insert to the pocket of the tool holder, so that two adjacent facets are placed against corresponding flat contact areas formed on the inner surface of the holder.
Each two adjacent flat contact areas form an obtuse angle therebetween and each flat surface forms an acute angle with the axis of the insert, which is slightly larger than the abutment angle formed by the locating surface, so that the initial contact between the insert and the inner surface of the receiving pocket occurs adjacent the upper ends of the facets and these latter are pressed into the upper locating wall, when the insert is screwed into place by its clamping screw.
In this device, too, the torque-resistance capability is limited, particularly because of the limited surface of the contacting areas of the insert and the receiving pocket, and because the pressure angle (defined below) is close to 90.degree..
There is therefore a need for cutting inserts with circular or polygonal cutting edges which provide appropriately oriented abutment surfaces for opposing significant torques while allowing a relatively large number of indexing positions. There is also a need for tool holders for mounting such cutting inserts.